The present invention generally relates to silica scale inhibitors. More specifically, the present invention relates to a method for inhibiting the formation and deposition of silica and silicate compounds in water systems with water-soluble polyamides.
In many parts of the world, amorphous silica scales cause significant fouling problems when industrial waters contain high quantities of silica. For the most part, high quantities of silica means that the industrial waters contain at least 5 ppm and up to about 500 ppm dissolved silica and may contain higher quantities of silica either in dissolved, dispersed or colloidal forms.
The solubility of silica adversely limits the efficient use of water in industrial applications, such as cooling, boiler, geothermal, reverse osmosis and papermaking. Specifically, water treatment operations are limited because the solubility of silica at about 150 ppm can be exceeded when minerals are concentrated during processing. This can result in the precipitation and deposition of amorphous silica and silicates with consequential loss of equipment efficiency. Moreover, the accumulation of silica on internal surfaces of water treatment equipment, such as boilers, cooling, and purification systems, reduces heat transfer and fluid flow through heat exchange tubes and membranes.
Once the silica scale forms on water treatment equipment, the removal of such scale is very difficult and costly. With high silica water, therefore, cooling and reverse osmosis systems typically operate at low water-use efficiency to assure that the solubility of silica is not exceeded. Under these conditions, however, reverse osmosis systems must limit their pure water recovery rate and cooling systems must limit water recycling. In both cases, water discharge volumes are large.
Various additives have been employed over the years to inhibit silica deposition. The current technologies for silica scale control in industrial cooling systems involve the use of either colloidal silica dispersants or silica polymerization inhibitors. Dispersant technologies have shown little activity, being able to stabilize only slight increases of total silica in a tower. For instance, by feeding a dispersant, silica levels may increase from 150-200 to 180-220 ppm, which is often an undetectable increase in silica cycles.
On the other hand, silica polymerization inhibitors have shown to be more effective against silica scale deposition. For example, U.S. Pat. No. 4,532,047 to Dubin relates to the use of a water-soluble low molecular weight polypolar organic compound for inhibiting amorphous silica scale formation on surfaces in contact with industrial waters. Likewise, U.S. Pat. No. 5,658,465 to Nicholas et al relates to the use of polyoxazoline as a silica scale inhibition technology. These polymerization inhibitors have allowed for increases in soluble silica to greater than 300 ppm without scale formation.